In recent years, surface-treated steel sheets obtained by rustproofing base steel sheets, particularly galvanized steel sheets which can be manufactured at low cost and which are excellent in rust resistance, have been used in the fields of automobiles, home appliances, building materials, and the like. In particular, European and American carmakers are attempting to enhance rust prevention performance by the use of galvanized steel sheet which can be readily increased in coating thickness. High demand for automotive steel sheets can be expected in the East Asian region, which is remarkable in economic growth.
In the case of an automotive steel sheet strictly required to have good workability, if coating quality including post-pressing anti-impact adhesion and post-working, post-painting corrosion resistance is not good, then durability cannot be maintained. Hitherto, no galvanized steel sheet with sufficient coating quality has been capable of being provided.
In particular, severe workability and the rust resistance of a worked portion are required for a so-called high-strength steel sheet used as a strength member. Therefore, the worked portion is required to have excellent coating quality. Furthermore, such a high-strength steel sheet is required to have spot weldability from the viewpoint that a steel sheet formed in a carmaker, particularly a high-strength steel sheet used for automotive weight reduction, is assembled for automobile bodies.
Patent Literature 1 discloses a method for manufacturing a galvanized steel sheet excellent in slidability in pressing by regulating the content of Al in a coating layer and the amount of Al at the interface between the coating layer and a steel sheet. However, a technique described in Patent Literature 1 does not sufficiently take into account coating quality including the anti-impact adhesion and post-painting corrosion resistance of a worked portion, coating appearance, or spot weldability and therefore is required to be further improved.